[https://www.gnu.org/software/grub/ GRUB2] is the next generation of the GRand Unified Bootloader (GRUB). GRUB2 is derived from [http://www.nongnu.org/pupa/ PUPA] which was a research project to investigate the next generation of GRUB. GRUB2 has been rewritten from scratch to clean up everything and provide modularity and portability [https://www.gnu.org/software/grub/grub-faq.html#q1].

[https://www.gnu.org/software/grub/ GRUB2] is the next generation of the GRand Unified Bootloader (GRUB). GRUB2 is derived from [http://www.nongnu.org/pupa/ PUPA] which was a research project to investigate the next generation of GRUB. GRUB2 has been rewritten from scratch to clean up everything and provide modularity and portability [https://www.gnu.org/software/grub/grub-faq.html#q1].

GRUB2 is the next generation of the GRand Unified Bootloader (GRUB). GRUB2 is derived from PUPA which was a research project to investigate the next generation of GRUB. GRUB2 has been rewritten from scratch to clean up everything and provide modularity and portability [1].

In brief, the bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the Linux kernel. The kernel, in turn, initializes the rest of the operating system.

Notes for current GRUB Legacy users

Upgrade from GRUB Legacy to GRUB(2) is the much same as fresh installing GRUB(2)which is covered below.

There are differences in the commands of GRUB and GRUB2. Familiarize yourself with GRUB2 commands before proceeding (e.g. "find" has been replaced with "search").

GRUB2 is now modular and no longer requires "stage 1.5". As a result, the bootloader itself is limited -- modules are loaded from the hard drive as needed to expand functionality (e.g. for LVM or RAID support).

Device naming has changed between GRUB and GRUB2. Partitions are numbered from 1 instead of 0 while drives are still numbered from 0, and prefixed with partition-table type. For example, /dev/sda1 would be referred to as (hd0,msdos1) (for MBR) or (hd0,gpt1) (for GPT) using GRUB2.

Preliminary Requirements for GRUB2

BIOS systems

GRUB2 in BIOS-GPT configuration requires a BIOS Boot Partition to embed its core.img in the absence of post-MBR gap in GPT partitioned systems (which is taken over by the GPT Primary Header and Primary Partition table). This partition is used by GRUB2 only in BIOS-GPT setups. No such partition type exists in case of MBR partitioning (at least not for GRUB2). This partition is also not required if the system is UEFI based, as no embedding of bootsectors takes place in that case. Syslinux does not require this partition.

For a BIOS-GPT configuration, create a 2 MiB partition using cgdisk or GNU Parted with no filesystem. The location of the partition in the partition table does not matter but it should be within the first 2 TiB region of the disk. It is advisable to put it somewhere in the beginning of the disk before the /boot partition. Set the partition type to "EF02" in cgdisk or set <BOOT_PART_NUM> bios_grub on in GNU Parted.

Note: This partition should be created before grub-install or grub-setup is run or before the Install Bootloader step of the Archlinux installer (if GRUB2 BIOS is selected as bootloader).

Usually the post-MBR gap (after the 512 byte MBR region and before the start of the 1st partition) in many MBR (or msdos disklabel) partitioned systems is 31 KiB when DOS compatibility cylinder alignment issues are satisfied in the partition table. However a post-MBR gap of about 1 to 2 MiB is recommended to provide sufficient room for embedding GRUB2's core.img (FS#24103). It is advisable to use a partitioner which supports 1 MiB partition alignment to obtain this space as well as satisfy other non-512 byte sector issues (which are unrelated to embedding of core.img).

MBR partitioning has better support in other operating systems, such as Microsoft Windows (up to Windows 7) and Haiku, than GPT partitioning. If you dual boot another operating system, consider using MBR partitioning.

Warning: Create the 2MiB partition mentioned above BEFORE you convert to GPT. If you do not, gparted will not resize your boot partition to allow its creation, and when you reboot GRUB2 will not know where to look.

Only 446 bytes of the MBR contain boot code, the next 64 contain the partition table. If you do not want to overwrite your partition table when restoring, it is strongly advised to backup only the MBR boot code:

Install grub-bios package

The GRUB(2) packages can be installed with pacman (and will replace grub-legacy or grub, if it is installed):

# pacman -S grub-bios

Note: Simply installing the package won't update the /boot/grub/i386-pc/core.img file and the GRUB(2) modules in /boot/grub/i386-pc. You need to update them manually using grub-install as explained below.

where /dev/sda is the destination of the installation (in this case the MBR of the first SATA disk). If you use LVM for your /boot, you can install GRUB2 on multiple physical disks.

Note: Without --target or --directory option, grub-install cannot determine for which firmware grub(2) is being installed. In such cases grub-install will show source_dir doesn't exist. Please specify --target or --directory message. Also, --target=i386-pc is correct for 64-bit systems, as well.

The --no-floppy tells grub-bios utilities not to search for any floppy devices which reduces the overall execution time of grub-install on many systems (it will also prevent the issue below from occurring). Otherwise you get an error that looks like this:

grub-probe: error: Cannot get the real path of '/dev/fd0'
Auto-detection of a filesystem module failed.
Please specify the module with the option '--modules' explicitly.

Note: --no-floppy has been removed from grub-install in 2.00~beta2 upstream release, and replaced with --allow-floppy.

Warning: Make sure to check the /boot directory if you use the latter. Sometimes the boot-directory parameter creates another /boot folder inside of /boot. A wrong install would look like: /boot/boot/grub/.

Install to Partition or Partitionless Disk

Note: grub-bios (any version - including upstream Bazaar repo) does not encourage installation to a partition boot sector or a partitionless disk like GRUB Legacy or Syslinux does. This kind of setup is prone to breakage, especially during updates, and is not supported by Arch devs.

To set up grub-bios to a partition boot sector, to a partitionless disk (also called superfloppy) or to a floppy disk, run (using for example /dev/sdaX as the /boot partition):

You need to use the --force option to allow usage of blocklists and should not use --grub-setup=/bin/true (which is similar to simply generating core.img).

grub-install will give out warnings like which should give you the idea of what might go wrong with this approach:

/sbin/grub-setup: warn: Attempting to install GRUB to a partitionless disk or to a partition. This is a BAD idea.
/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists.
However, blocklists are UNRELIABLE and their use is discouraged.

Without --force you may get the below error and grub-setup will not setup its boot code in the partition boot sector:

/sbin/grub-setup: error: will not proceed with blocklists

With --force you should get:

Installation finished. No error reported.

The reason why grub-setup does not by default allow this is because in case of partition or a partitionless disk is that grub-bios relies on embedded blocklists in the partition bootsector to locate the /boot/grub/i386-pc/core.img file and the prefix dir /boot/grub. The sector locations of core.img may change whenever the filesystem in the partition is being altered (files copied, deleted etc.). For more info see https://bugzilla.redhat.com/show_bug.cgi?id=728742 and https://bugzilla.redhat.com/show_bug.cgi?id=730915.

The workaround for this is to set the immutable flag on /boot/grub/i386-pc/core.img (using chattr command as mentioned above) so that the sector locations of the core.img file in the disk is not altered. The immutable flag on /boot/grub/i386-pc/core.img needs to be set only if grub-bios is installed to a partition boot sector or a partitionless disk, not in case of installation to MBR or simple generation of core.img without embedding any bootsector (mentioned above).

Generate core.img alone

To populate the /boot/grub directory and generate a /boot/grub/i386-pc/core.img file without embedding any grub-bios bootsector code in the MBR, post-MBR region, or the partition bootsector, add --grub-setup=/bin/true to grub-install:

Multiboot in BIOS

Boot Microsoft Windows installed in BIOS-MBR mode

Note: GRUB(2) supports booting bootmgr directly and chainload of partition boot sector is no longer required to boot Windows in a BIOS-MBR setup.

Warning: Take note that it is the SYSTEM PARTITION that has the bootmgr, not your "real" windows partition (C:), ie: when showing all UUID's with blkid it is the partition with LABEL="SYSTEM RESERVED" which is only about 100 mb big much like the boot partition for arch. See http://en.wikipedia.org/wiki/System_partition_and_boot_partition for some more info.

Find the UUID of the NTFS filesystem of the Windows's SYSTEM PARTITION where the bootmgr and its files reside. For example, if Windows bootmgr exists at /media/SYSTEM_RESERVED/bootmgr:

UEFI systems

Hardware-Specific UEFI Examples

It is well know that different motherboard manufactures implement UEFI differently. Users experiencing problems getting Grub/EFI to work properly are encouraged to share detailed steps for hardware-specific cases where UEFI booting does not work as described below. In an effort to keep the parent GRUB article neat and tidy, see the GRUB EFI Examples page for these special cases.

Install grub-uefi package

Note: Unless specified as EFI 1.x , EFI and UEFI terms are used interchangeably to denote UEFI 2.x firmware. Also unless stated explicitly, the instructions are general and not Mac specific. Some of them may not work or may be different in Macs. Apple's EFI implementation is neither a EFI 1.x version nor UEFI 2.x version but mixes up both. This kind of firmware does not fall under any one UEFI Specification version and is therefore not a standard UEFI firmware.

GRUB(2) UEFI bootloader is available in Arch Linux only from version 1.99~rc1. To install, first detect which UEFI firmware arch you have (either x86_64 or i386).

Depending on that, install the appropriate package

For 64-bit aka x86_64 UEFI firmware:

# pacman -S grub-efi-x86_64

For 32-bit aka i386 UEFI firmware:

# pacman -S grub-efi-i386

Note: Simply installing the package will not update the core.efi file and the GRUB(2) modules in the UEFI System Partition. You need to do this manually using grub-install as explained below.

Install grub-uefi boot files

Install to UEFI SYSTEM PARTITION

Note: The below commands assume you are using grub-efi-x86_64 (for grub-efi-i386 replace x86_64 with i386 in the below commands).

Note: To do this, you need to boot using UEFI and not the BIOS. If you booted by just copying the ISO file to the USB drive, you will need to follow this guide or grub-install will show errors.

The UEFI system partition will need to be mounted at /boot/efi/ for the GRUB(2) install script to detect it:

# mkdir -p /boot/efi
# mount -t vfat /dev/sdXY /boot/efi

Install GRUB UEFI application to /boot/efi/EFI/arch_grub and its modules to /boot/grub/x86_64-efi (recommended) using:

Note: Without --target or --directory option, grub-install cannot determine for which firmware grub(2) is being installed. In such cases grub-install will show source_dir doesn't exist. Please specify --target or --directory message.

If you want to install grub(2) modules and grub.cfg at the directory /boot/efi/EFI/grub and the grubx64.efi application at /boot/efi/EFI/arch_grub (ie. all the grub(2) uefi files inside the UEFISYS partition itself) use:

The --efi-directory option mentions the mountpoint of UEFI SYSTEM PARTITION , --bootloader-id mentions the name of the directory used to store the grubx64.efi file and --boot-directory mentions the directory wherein the actual modules will be installed (and into which grub.cfg should be created).

The <efi-directory>/<EFI or efi>/<bootloader-id>/grubx64.efi is an exact copy of <boot-directory>/grub/x86_64-efi/core.efi.

Note: In GRUB 2.00, the grub-install option --efi-directory replaces --root-directory and the latter is deprecated.

Note: The options --efi-directory and --bootloader-id are specific to GRUB(2) UEFI.

In all the cases the UEFI SYSTEM PARTITION should be mounted for grub-install to install grubx64.efi in it, which will be launched by the firmware (using the efibootmgr created boot entry in non-Mac systems).

If you notice carefully, there is no <device_path> option (Eg: /dev/sda) at the end of the grub-install command unlike the case of setting up GRUB(2) for BIOS systems. Any <device_path> provided will be ignored by the install script as UEFI bootloaders do not use MBR or Partition boot sectors at all.

Create GRUB2 Standalone UEFI Application

It is possible to create a grubx64_standalone.efi application which has all the modules embeddded in a memdisk within the uefi application, thus removing the need for having a separate directory populated with all the GRUB2 uefi modules and other related files. This is done using the grub-mkstandalone command which is included in grub-common >= 1:1.99-6 package.

The easiest way to do this would be with the install command already mentioned before, but specifying the modules to include. For example:

The grubx64_standalone.efi file expects grub.cfg to be within its $prefix which is (memdisk)/boot/grub. The memdisk is embedded within the efi app. The grub-mkstandlone script allow passing files to be included in the memdisk image to be as the arguments to the script (in <any extra files you want to include>).

If you have the grub.cfg at /home/user/Desktop/grub.cfg, then create a temporary /home/user/Desktop/boot/grub/ directory, copy the /home/user/Desktop/grub.cfg to /home/user/Desktop/boot/grub/grub.cfg, cd into /home/user/Desktop/boot/grub/ and run:

The reason to cd into /home/user/Desktop/boot/grub/ and to pass the file path as boot/grub/grub.cfg (notice the lack of a leading slash - boot/ vs /boot/ ) is because dir1/dir2/file is included as (memdisk)/dir1/dir2/file by the grub-mkstandalone script.

If you pass /home/user/Desktop/grub.cfg the file will be included as (memdisk)/home/user/Desktop/grub.cfg. If you pass /home/user/Desktop/boot/grub/grub.cfg the file will be included as (memdisk)/home/user/Desktop/boot/grub/grub.cfg. That is the reason for cd'ing into /home/user/Desktop/boot/grub/ and passing boot/grub/grub.cfg, to include the file as (memdisk)/boot/grub/grub.cfg, which is what grub.efi expects the file to be.

Multiboot in UEFI

Chainload Microsoft Windows x86_64 UEFI-GPT

Find the UUID of the FAT32 filesystem in the UEFI SYSTEM PARTITION where the Windows UEFI Bootloader files reside. For example, if Windows bootmgfw.efi exists at /boot/efi/EFI/Microsoft/Boot/bootmgfw.efi (ignore the upper-lower case differences since that is immaterial in FAT filesystem):

Configuration

You can also choose to automatically generate or manually edit grub.cfg.

Note: For EFI systems, if GRUB2 was installed with the --boot-directory option set, the grub.cfg file must be placed in the same directory as grubx64.efi. Otherwise, the grub.cfg file goes in /boot/grub/, just like in the BIOS version of GRUB2.

Automatically generating using grub-mkconfig (Recommended)

The GRUB2 menu.lst equivalent configuration files are /etc/default/grub and /etc/grub.d/*. grub-mkconfig uses these files to generate grub.cfg. By default the script outputs to stdout. To generate a grub.cfg file run the command:

# grub-mkconfig -o /boot/grub/grub.cfg

/etc/grub.d/10_linux is set to automatically add menu items for Arch linux that work out of the box, to any generated configuration. Other operating systems may need to be added manually to /etc/grub.d/40_custom or /boot/grub/custom.cfg

Additional arguments

To pass custom additional arguments to the Linux image, you can set the GRUB_CMDLINE_LINUX variable in /etc/default/grub.

For example, use GRUB_CMDLINE_LINUX="resume=/dev/sdaX" where sdaX is your swap partition to enable resume after hibernation.

You can also use GRUB_CMDLINE_LINUX="resume=/dev/disk/by-uuid/${swap_uuid}", where ${swap_uuid} is the UUID of your swap partition.

Multiple entries are separated by spaces within the double quotes. So, for users who want both resume and systemd it would look like this:
GRUB_CMDLINE_LINUX="resume=/dev/sdaX init=/usr/lib/systemd/systemd"

Manually creating grub.cfg

Warning: Editing this file is strongly not recommended. The file is generated by the grub-mkconfig command, and it is best to edit your /etc/default/grub or one of the scripts in the /etc/grub.d folder.

Dual-booting

Note: If you want GRUB2 to automatically search for other systems, you may wish to install os-prober.

Using grub-mkconfig

The best way to add other entries is editing the /etc/grub.d/40_custom or /boot/grub/custom.cfg . The entries in this file will be automatically added when running grub-mkconfig.
After adding the new lines, run:

With FreeBSD

Requires that FreeBSD is installed on a single partition with UFS. Assuming it is installed on sda4:

menuentry "FreeBSD" {
set root=(hd0,4)
chainloader +1
}

With Windows

This assumes that your Windows partition is sda3. Remember you need to point set root and chainloader to the system reserve partition that windows made when it installed, not the actual partition windows is on. This example works if your system reserve partition is sda3.

If the Windows bootloader is on an entirely different hard drive than GRUB, it may be necessary to trick Windows into believing that it is the first hard drive. This was possible with drivemap. Assuming GRUB is on hd0 and Windows is on hd2, you need to add the following after set root:

drivemap -s hd0 hd2

With Windows via EasyBCD and NeoGRUB

Since EasyBCD's NeoGRUB currently does not understand the GRUB2 menu format, chainload to it by replacing the contents of your C:\NST\menu.lst file with lines similar to the following:

Visual Configuration

In GRUB2 it is possible, by default, to change the look of the menu. Make sure to initialize, if not done already, GRUB2 graphical terminal, gfxterm, with proper video mode, gfxmode, in GRUB2. This can be seen in the section #Correct_GRUB2_No_Suitable_Mode_Found_Error. This video mode is passed by GRUB2 to the linux kernel via 'gfxpayload' so any visual configurations need this mode in order to be in effect.

Setting the framebuffer resolution

GRUB2 can set the framebuffer for both GRUB2 itself and the kernel. The old vga= way is deprecated. The preferred method is editing /etc/default/grub as the following sample:

GRUB_GFXMODE=1024x768x32
GRUB_GFXPAYLOAD_LINUX=keep

To generate the changes, run:

grub-mkconfig -o /boot/grub/grub.cfg

The gfxpayload property will make sure the kernel keeps the resolution.

Note: If this example does not work for you try to replace gfxmode="1024x768x32" by vbemode="0x105". Remember to replace the specified resolution with one suitable for your screen.

Note: To show all the modes you can use # hwinfo --framebuffer (hwinfo is available in [community]), while at GRUB2 prompt you can use the vbeinfo command.

If this method does not work for you, the deprecated vga= method will still work. Just
add it next to the "GRUB_CMDLINE_LINUX_DEFAULT=" line in /etc/default/grub
for eg: "GRUB_CMDLINE_LINUX_DEFAULT="quiet splash vga=792" will give you a 1024x768 resolution.

You can choose one of these resolutions: 640×480, 800×600, 1024×768, 1280×1024, 1600×1200, 1920×1200

915resolution hack

Some times for Intel graphic adapters neither # hwinfo --framebuffer nor vbeinfo will show you the desired resolution. In this case you can use 915resolution hack. This hack will temporarily modify video BIOS and add needed resolution. See 915resolution's home page

In the following I will proceed with the example for my system. Please adjust the recipe for your needs. First you need to find a video mode which will be modified later. For that, run 915resolution in GRUB2 command shell:

Next, our purpose is to overwrite mode 30. (You can choose what ever mode you want.) In the file /etc/grub.d/00_header just before the set gfxmode=${GRUB_GFXMODE} line insert:

915resolution 30 1440 900

Here we are overwriting the mode 30 with 1440x900 resolution. Lastly we need to set GRUB_GFXMODE as described earlier, regenerate GRUB2 configuration file and reboot to test changes:

# grub-mkconfig -o /boot/grub/grub.cfg
# reboot

Background image and bitmap fonts

GRUB2 comes with support for background images and bitmap fonts in pf2 format. The unifont font is included in the grub-common package under the filename unicode.pf2, or, as only ASCII characters under the name ascii.pf2.

Note: If you have installed GRUB on a separate partition, /boot/grub/myimage becomes /grub/myimage.

To generate the changes and add the information into grub.cfg, run:

grub-mkconfig -o /boot/grub/grub.cfg

If adding the splash image was successful, the user will see "Found background image..." in the terminal as the command is executed.
If this phrase is not seen, the image information was probably not incorporated into the grub.cfg file.

If the image is not displayed, check:

The path and the filename in /etc/default/grub are correct.

The image is of the proper size and format (tga, png, 8-bit jpg).

The image was saved in the RGB mode, and is not indexed.

The console mode is not enabled in /etc/default/grub.

The command grub-mkconfig must be executed to place the background image information into the /boot/grub/grub.cfg file.

Theme

Here is an example for configuring Starfield theme which was included in GRUB2 package.

Edit /etc/default/grub

GRUB_THEME="/usr/share/grub/themes/starfield/theme.txt"

Generate the changes:

grub-mkconfig -o /boot/grub/grub.cfg

If configuring the theme was successful, you'll see Found theme: /usr/share/grub/themes/starfield/theme.txt in the terminal.
Your splash image will usually not be displayed when using a theme.

Hidden menu

One of the unique features of GRUB2 is hiding/skipping the menu and showing it by holding Template:Keypress when needed. You can also adjust whether you want to see the timeout counter.

Edit /etc/default/grub as you wish. Here is an example where the comments from the beginning of the two lines have been removed to enable the feature, the timeout has been set to five seconds and to be shown to the user:

GRUB_HIDDEN_TIMEOUT=5
GRUB_HIDDEN_TIMEOUT_QUIET=false

and run:

grub-mkconfig -o /boot/grub/grub.cfg

Disable framebuffer

Users who use NVIDIA proprietary driver might wish to disable GRUB2's framebuffer as it can cause problems with the binary driver.

To disable framebuffer, edit /etc/default/grub and uncomment the following line:

GRUB_TERMINAL_OUTPUT=console

and run:

grub-mkconfig -o /boot/grub/grub.cfg

Other Options

LVM

If you use LVM for your /boot, add the following before menuentry lines:

RAID

GRUB2 provides convenient handling of RAID volumes. You need to add insmod mdraid which allows you to address the volume natively. For example, /dev/md0 becomes:

set root=(md0)

whereas a partitioned RAID volume (e.g. /dev/md0p1) becomes:

set root=(md0,1)

Persistent block device naming

One naming scheme for Persistent block device naming is the use of globally unique UUIDs to detect partitions instead of the "old" /dev/sd*. Advantages are covered up in the above linked article.

Persistent naming via filesystem UUIDs are used by default in GRUB2.

Note: The /boot/grub.cfg file needs regeneration with the new UUID in /etc/default/grub every time a relevant filesystem is resized or recreated. Remember this when modifying partitions & filesystems with a Live-CD.

Whether to use UUIDs is controlled by an option in /etc/default/grub:

# GRUB_DISABLE_LINUX_UUID=true

Either way, do not forget to generate the changes:

# grub-mkconfig -o /boot/grub/grub.cfg

Using Labels

It is possible to use labels, human-readable strings attached to filesystems, by using the --label option to search. First of all, label your existing partition:

Recall previous entry

GRUB2 can remember the last entry you booted from and use this as the default entry to boot from next time. This is useful if you have multiple kernels (i.e., the current Arch one and the LTS kernel as a fallback option) or operating systems. To do this, edit /etc/default/grub and change the setting of GRUB_DEFAULT:

GRUB_DEFAULT=saved

This ensures that GRUB will default to the saved entry. To enable saving the selected entry, add the following line to /etc/default/grub:

Security

If you want to secure GRUB2 so it is not possible for anyone to change boot parameters or use the command line, you can add a user/password combination to GRUB2's configuration files. To do this, run the command grub-mkpasswd-pbkdf2. Enter a password and confirm it. The output will look like this:

Booting an ISO Directly From GRUB2

Edit /etc/grub.d/40_custom or /boot/grub/custom.cfg to add an entry for the target ISO. When finished, update the GRUB menu as with the usual grub-mkconfig -o /boot/grub/grub.cfg (as root).

Arch ISO

Note: Be sure to adjust the hdX,Y in the third line to point to the correct disk/partition number of the isofile. Also adjust the img_dev line to match this same location. However, if booting the ISO from USB on a computer which also has one internal HDD, then it needs to be hd0,Y with sdbY, instead of sdaY.

Other ISOs

Using the command shell

Since the MBR is too small to store all GRUB2 modules, only the menu and a few basic commands reside there. The majority of GRUB2 functionality remains in modules in /boot/grub, which are inserted as needed. In error conditions (e.g. if the partition layout changes) GRUB2 may fail to boot. When this happens, a command shell may appear.

GRUB2 offers multiple shells/prompts. If there is a problem reading the menu but the bootloader is able to find the disk, you will likely be dropped to the "normal" shell:

sh:grub>

If there is a more serious problem (e.g. GRUB cannot find required files), you may instead be dropped to the "rescue" shell:

grub rescue>

The rescue shell is a restricted subset of the normal shell, offering much less functionality. If dumped to the rescue shell, first try inserting the "normal" module, then starting the "normal" shell:

Using the rescue console

See #Using the command shell first. If unable to activate the standard shell, one possible solution is to boot using a live CD or some other rescue disk to correct configuration errors and reinstall GRUB. However, such a boot disk is not always available (nor necessary); the rescue console is surprisingly robust.

The available commands in GRUB rescue include insmod, ls, set, and unset. This example uses set and insmod. set modifies variables and insmod inserts new modules to add functionality.

Before starting, the user must know the location of their /boot partition (be it a separate partition, or a subdirectory under their root):

grub rescue> set prefix=(hdX,Y)/boot/grub

where X is the physical drive number and Y is the partition number.

To expand console capabilities, insert the linux module:

grub rescue> insmod (hdX,Y)/boot/grub/linux.mod

Note: With a separate boot partition, omit /boot from the path, (i.e. type set prefix=(hdX,Y)/grub and insmod (hdX,Y)/grub/linux.mod).

This introduces the linux and initrd commands, which should be familiar (see #Configuration).

After successfully booting the Arch Linux installation, users can correct grub.cfg as needed and then reinstall GRUB2.

to reinstall GRUB2 and fix the problem completely, changing /dev/sda if needed. See #Bootloader installation for details.

Combining the use of UUIDs and basic scripting

If you like the idea of using UUIDs to avoid unreliable BIOS mappings or are struggling with GRUB's syntax, here is an example boot menu item that uses UUIDs and a small script to direct GRUB to the proper disk partitions for your system. All you need to do is replace the UUIDs in the sample with the correct UUIDs for your system. The example applies to a system with a boot and root partition. You will obviously need to modify the GRUB configuration if you have additional partitions:

menuentry "Arch Linux 64" {
# Set the UUIDs for your boot and root partition respectively
set the_boot_uuid=ece0448f-bb08-486d-9864-ac3271bd8d07
set the_root_uuid=c55da16f-e2af-4603-9e0b-03f5f565ec4a
# (Note: This may be the same as your boot partition)
# Get the boot/root devices and set them in the root and grub_boot variables
search --fs-uuid --no-floppy --set=root $the_root_uuid
search --fs-uuid --no-floppy --set=grub_boot $the_boot_uuid
# Check to see if boot and root are equal.
# If they are, then append /boot to $grub_boot (Since $grub_boot is actually the root partition)
if [ $the_boot_uuid == $the_root_uuid] ; then
set grub_boot=$grub_boot/boot
fi
# $grub_boot now points to the correct location, so the following will properly find the kernel and initrd
linux ($grub_boot)/vmlinuz-linux root=/dev/disk/by-uuid/$uuid_os_root ro
initrd ($grub_boot)/initramfs-linux.img
}

Troubleshooting

Any troubleshooting should be added here.

Enable GRUB2 debug messages

Add:

set pager=1
set debug=all

to grub.cfg.

Correct GRUB2 No Suitable Mode Found Error

If you get this error when booting any menuentry:

error: no suitable mode found
Booting however

Then you need to initialize GRUB2 graphical terminal (gfxterm) with proper video mode (gfxmode) in GRUB2. This video mode is passed by GRUB2 to the linux kernel via 'gfxpayload'. In case of UEFI systems, if the GRUB2 video mode is not initialized, no kernel boot messages will be shown in the terminal (atleast until KMS kicks in).

Copy /usr/share/grub/unicode.pf2 to ${GRUB2_PREFIX_DIR} (/boot/grub/ in case of BIOS and UEFI systems). If GRUB2 UEFI was installed with --boot-directory=/boot/efi/EFI set, then the directory is /boot/efi/EFI/grub/:

# cp /usr/share/grub/unicode.pf2 ${GRUB2_PREFIX_DIR}

If /usr/share/grub/unicode.pf2 does not exist, install bdf-unifont, create the unifont.pf2 file and then copy it to ${GRUB2_PREFIX_DIR}:

# grub-mkfont -o unicode.pf2 /usr/share/fonts/misc/unifont.bdf

Then, in the grub.cfg file, add the following lines to enable GRUB2 to pass the video mode correctly to the kernel, without of which you will only get a black screen (no output) but booting (actually) proceeds successfully without any system hang.

BIOS systems:

insmod vbe

UEFI systems:

insmod efi_gop
insmod efi_uga

After that add the following code (common to both BIOS and UEFI):

insmod font

if loadfont ${prefix}/fonts/unicode.pf2
then
insmod gfxterm
set gfxmode=auto
set gfxpayload=keep
terminal_output gfxterm
fi

As you can see for gfxterm (graphical terminal) to function properly, unicode.pf2 font file should exist in ${GRUB2_PREFIX_DIR}.

msdos-style error message

grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding won't be possible!
grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists.
However, blocklists are UNRELIABLE and its use is discouraged.
grub-setup: error: If you really want blocklists, use --force.

This error may occur when you try installing GRUB2 in a VMware container. Read more about it here. It happens when the first partition starts just after the MBR (block 63), without the usual space of 1 MiB (2048 blocks) before the first partition. Read #MBR_aka_msdos_partitioning_specific_instructions

UEFI GRUB2 drops to shell

If GRUB loads but drops you into the rescue shell with no errors, it may be because of a missing or misplaced grub.cfg. This will happen if GRUB2 UEFI was installed with --boot-directory and grub.cfg is missing OR if the partition number of the boot partition changed (which is hard-coded into the grubx64.efi file).

UEFI GRUB2 not loaded

In some cases the EFI may fail to load GRUB correctly. Provided everything is set up correctly, the output of:

If everything works correctly, the EFI would now automatically load GRUB.

If the screen only goes black for a second and the next boot option is tried afterwards, according to this post, moving GRUB to the partition root can help. The boot option has to be deleted and recreated afterwards. The entry for GRUB should look like this then: